The invention relates, generally, to telecommunications networks and, more particularly, to a compressed voice communication system that operates at an extremely low data rate.
It will be understood that speech is transmitted in a variety of different types of networks such as the public switched telephone network, satellite communications systems, submarine communications systems, cellular telephone systems, personal communications systems and the like. In the public switched telephone network, almost all speech is band-limited to the 200 to 3,800 hertz (Hz) range. This band-limited signal is often referred to as telephone bandwidth speech. Telephone bandwidth speech is sampled 8,000 times per second (8 Khz) and quantized with an 8-bit logarithm's quantizer, making the normal bit rate used for transmission in the public switched telephone network 64 kilobits per second (Kbps). The 64 Kbps data rate used in the public switched telephone network accurately represents speech with minimal degradation in speech quality.
While the 64 Kbps data rate is adequately supported in the public switched telephone network where bandwidth limitations are generally not a problem, other applications and/or telecommunications systems are not able to support this relatively high data rate. For example, satellite communications systems, cellular telephone systems, submarine communications systems and other communications systems that have limited bandwidth all require data rates lower than the 64 Kbps data rate of the public switched telephone network. Moreover, even though the public switched telephone network adequately supports the 64 Kbps data rate, the ability to transmit speech at much lower data rates with minimum quality degradation would greatly increase the efficiencies of the public switched telephone network for some applications. As a result, numerous speech compression techniques have been developed for compressing speech into a signal having a data rate lower than the 64 Kpbs telephone bandwidth speed.
Typically, speech compression is obtained using speech coders, as are known in the art, that exploit the natural redundancies and pauses in speech and the properties of human hearing to compress speech into a relatively low bit rate data stream. The degree of compression is measured by how much the bit rate is lowered from the 64 Kbps telephone bandwidth speech. International standards exist for coders operating at 40, 32, 24 and 16 Kbps and are planned for rates as low as 4 Kbps. Moreover, regional cellular standards span the data rate range from 13 to 3.45 Kbps. While existing speech compression techniques have reduced the bandwidth required for transmitting speech, the bit rate of the compressed speech is on the same order of magnitude as the bit rate of telephone bandwidth speech. An even greater reduction in bit rate is desired for such applications as video telephony where the bandwidth needed for the video portion limits the bandwidth available for speech, satellite communication systems where capacity can only be increased by reducing bit rate, cellular and internet applications where limited bandwidth is in increasingly high demand and any other bandwidth sensitive applications.
Thus, an improved communications system for compressing and transmitting speech is desired.